The present invention relates to a machine for machining workpieces by means of a laser beam. The process, consisting in evaporating material from the surface of a workpiece by means of a focused laser beam, is now very widely used. In this type of machining, the laser beam is reflected by two steering mirrors, which can pivot very rapidly about two perpendicular axes, the rotation of said steering mirrors being controlled by a computer according to a raster lying in a plane. The method is widely used for engraving patterns on plane surfaces. The high-power laser beam is produced by a laser source, at the outlet of which the beam is conducted right to the surface to be treated by means of optical fibres and a series of static or rotary mirrors and other optical components. Most industrial applications are in the field of tooling and the manufacture of moulds in which it is necessary to engrave the surface of certain workpieces with very fine control and precision. Material is removed by sudden heating of the area illuminated by the beam, causing a parcel of material to evaporate. The final diameter of the beam when it reaches the workpiece to be machined is less than 50 μm. The operation takes place in successive layers. The laser beam scans the workpiece within a polygon having dimensions limited to a few centimeters using a scanning device commonly referred to as a galvanometric scanner or “galvo-scanner”, consisting of two steering mirrors that can pivot very rapidly about two perpendicular axes. The polygons are defined in such a way that the surface to be treated lies within the focal tolerance of the laser beam when the latter treats the surface of a given polygon, and thus ensures machining uniformity. The two steering mirrors therefore can cover only a limited angular amplitude. The limited scanning amplitude requires the laser head to be periodically repositioned and reoriented about the workpiece to be treated. To treat a workpiece of any 3D geometry, it will be necessary to have available an at least five-axis machine. Furthermore, the machine will incorporate geometric measurement sensors, a touch probe and a camera for example, which are activated so that the laser head is precisely positioned and oriented relative to the workpiece to be machined.
Material removal takes place on all kinds of three-dimensional surfaces, including in concave regions. The method for producing a texture on any 3D surface consists in particular in segmenting the surface into a plurality of restricted partial surfaces within adjacent polygons.
In the thesis entitled “Near-net-shape laser beam structuring for plastic injection moulds” presented by Johannes Mario Kordt at the Rheinisch-Westfälische Technische Hochschule Aachen, a similar engraving machine is described in Section 5.1. The machine, based on a five-axis architecture of the MIKRON-HSM 600 U type, was adapted for treating workpieces weighing about 100 kg and having dimensions limited to a volume of 250×250×250 mm3. On one side, the workpiece to be machined is fixed on a worktable that pivots and moves along two perpendicular rotary axes B and C and one linear axis X. On the other side, the laser scanning head is supported by a linear shaft Z, the shaft itself moving along a final linear axis Y. Such a machine structure is not an ideal arrangement for machining heavy workpieces having a mass in excess of 200 kg, because of the flexibility of the cradle. Moreover, the size of the workpieces is limited by the size of the cradle.